An electrolytic capacitor has a characteristic feature in that it has a small size and a large electrostatic capacity, and it is widely used in low frequency filter and bypass. The electrolytic capacitor generally has a structure comprising an anode foil and a cathode foil being spirally wound together with a separator disposed between the anode foil and the cathode foil, and the resultant assembly is contained and sealed in a casing. As an anode foil, a metal such as aluminum or tantalum, having formed thereon an insulating oxide film as a dielectric layer is used. As a cathode foil, an etched aluminum foil is generally used. The separator disposed between the anode and the cathode is impregnated with an electrolyte and functions as a true cathode. Therefore, the electrolyte is an important constituent that directly affects the properties of the electrolytic capacitor.
Recently, as an electrolyte for electrolytic capacitor, having high electrolytic conductivity and excellent thermal stability as well as high voltage proof, an electrolyte for electrolytic capacitor containing a tetrafluoroaluminate ion has been proposed (see, for example, patent document 1).
However, in studies made on the electrolytic capacitor using the electrolyte a containing tetrafluoroaluminate ion, it has been found that the electrolytic capacitor has not only the problem of electrolyte leakage from the cathode, which has conventionally been pointed out, but also a problem of electrolyte leakage from the anode in the strict meaning, which has conventionally been considered not to occur.    Patent document 1: Japanese Unexamined Patent Publication No. 2003-142346